Back in October of last year, I posted a note here about a new study that examined historical records for evidence of global warming. The study, which confirmed previous results that the global average temperature had increased by 1°C since 1950, was somewhat noteworthy, because it was led by a self-described former global warming skeptic, Prof. Richard A. Muller, who is a professor of physics at the University of California, Berkeley, and a Faculty Senior Scientist at the Lawrence Berkeley National Laboratory. The research work, which is available at the BerkeleyEarth.orgproject site, examined some potential methodological problems with earlier research — notably the “urban heat island” hypothesis, and the questionable quality of some of the historical data — and found that correcting for them did not have any significant effect on the result. (My earlier post, linked above, has a more complete discussion of the research.) That study, though, did not address the question of why the warming was occurring.

Now Prof. Muller has published an Op-Ed column in the New York Times, in which he describes some new research that provides some significant evidence that the human-caused increase in atmospheric “greenhouse gases” (such as carbon dioxide and methane) is the cause of the warming.

Last year, following an intensive research effort involving a dozen scientists, I concluded that global warming was real and that the prior estimates of the rate of warming were correct. I’m now going a step further: Humans are almost entirely the cause.

The historical pattern observed by the team (the data are also available at the project web site) shows some expected short temperature dips, following large volcanic eruptions, which eject large amounts of particulate matter into the air, thereby shading the Earth’s surface from some solar radiation. There are also some small, short-term variations in temperatures that can be attributed to fluctuations in ocean currents, such as El Niño and the Gulf Stream. But there was only one indicator that was strongly correlated with the long-term temperature trend.

We tried fitting the shape to simple math functions (exponentials, polynomials), to solar activity and even to rising functions like world population. By far the best match was to the record of atmospheric carbon dioxide, measured from atmospheric samples and air trapped in polar ice.

Prof. Muller notes that their historical data covers a long enough period that by analyzing it in connection with sunspot data, changes in solar activity can be essentially eliminated as a cause of global warming.

… our data argues strongly that the temperature rise of the past 250 years cannot be attributed to solar changes. This conclusion is, in retrospect, not too surprising; we’ve learned from satellite measurements that solar activity changes the brightness of the sun very little.

Prof. Muller further notes, correctly, that the strong correlation observed between carbon dioxide levels and temperature increases does not prove that the latter are caused by the former. However, any posited alternative explanation should explain the historical record at least as well if it is to be taken seriously. (There is, in the greenhouse effect, a plausible physical / chemical mechanism by which increased carbon dioxide levels could cause warming.)

Prof. Muller also says (and I agree) that many of the claims made about global warming are “speculative, exaggerated, or just plain wrong”. The real point, though, is that there is more and more evidence that the average temperature on Earth is rising, and that it is being caused as a by-product of human activity.